Displacement speed for planing hull

[Adetheheat]

Is the displacement speed concept applicable to planing hulls or just displacement hulls?

 

[Tranona]

Any hull. It is the speed at which the bow and stern wave are equal and is determined by the waterline length. However planing hulls are often able to exceed this speed without planing because of the potential power available. Wasteful on fuel and creates a large wash. Many large planing powerboats spend a lot of time cruising at displacement speeds because of the fuel savings and greater comfort.

 

[NBs]

Hi,
In practice, yes.

If you think about fuel economy, you will find it about 2 knots below the theoretical hull speed.

 

[westernman]

Hi,
In practice, yes.

If you think about fuel economy, you will find it about 2 knots below the theoretical hull speed.

Hull speed in knots is reckoned to be 1.34 x sqrt(waterline length in feet).
For good fuel economy, you don't want to exceed 1.0 x sqrt(waterline length in feet).

Below 1.0 things are pretty linear. About 1.0 you start getting exponential. And at 1.34 the exponential is almost vertical.

Below 1.0, fuel consumption per nautical mile does not change much with speed.
I.e. the benefit of going at 0.5 instead of 1.0 is marginal (wind and current and other factors will tend to override any theoretical difference).

 

[Farmer Piles]

A really graphic illustration of westerman's great explanation is to look at the fuel consumption figures in the boat tests in MBY Magazine. At displacement speeds the fuel consumption can often double between 8 and 10 kts - look at their l/nm figures and the expected range. My mate found exactly this with his Swift Trawler 52, just by cruising at 9kts rather than 10 he almost halved his fuel consumption. Of course where this step occurs depends on hull length.
Similarly, I had a 19.5 ft sports boat with a Yamaha 115 and unless you ran at sub- 4kts she more efficient at 20kts.

 

[westernman]

A really graphic illustration of westerman's great explanation is to look at the fuel consumption figures in the boat tests in MBY Magazine. At displacement speeds the fuel consumption can often double between 8 and 10 kts - look at their l/nm figures and the expected range. My mate found exactly this with his Swift Trawler 52, just by cruising at 9kts rather than 10 he almost halved his fuel consumption. Of course where this step occurs depends on hull length.
Similarly, I had a 19.5 ft sports boat with a Yamaha 115 and unless you ran at sub- 4kts she more efficient at 20kts.

Waterline length (as far as I have googled) is 50ft.

So "hull speed" is about 9.5 knots.
So at 10 knots he is already digging a hole.

If he slowed down to 7 knots he will probably at least halve his consumption per mile again.

 

[B27]

Hull speed in knots is reckoned to be 1.34 x sqrt(waterline length in feet).
For good fuel economy, you don't want to exceed 1.0 x sqrt(waterline length in feet).

Below 1.0 things are pretty linear. About 1.0 you start getting exponential. And at 1.34 the exponential is almost vertical.

Below 1.0, fuel consumption per nautical mile does not change much with speed.
I.e. the benefit of going at 0.5 instead of 1.0 is marginal (wind and current and other factors will tend to override any theoretical difference).

Not sure that's always valid with power boat hulls.
If you've got a lot of transom immersed at rest/low speed, it's very draggy.

 

[westernman]

Not sure that's always valid with power boat hulls.
If you've got a lot of transom immersed at rest/low speed, it's very draggy.

At displacement speed a Swift trawler has brick shaped stern with the lower bit in the water.
At any displacement speed it is draggy. As you start digging a hole as you approach 9.5 knots, it will dig even more in the water.

So the difference between 1.0 x sqrt(LWL) vs 1.34 x sqrt(LWL) is likely to be even bigger for something like the Swift trawler than my boat which has a nice round stern. Also as I approach max hull speed, the effective LWL increases to take in more of the counter.

So as I said, slowing down from 9 knots to 7 knots is going to make a very big difference in fuel consumption per mile.

The hull speed of my boat is 9 knots. With 85HP I get at best 7.5 knots (clean bottom).
I need a lot of wind to get to 9 knots and full sail.

 

[jcwads]

Just on this topic. I plan to do some long journeys at displacement on my Targa 48. At what point would I become uneconomical? Assuming 8 or 9 knots is ok for economic cruising. IPS 600, D6 435 engines

 

[Sticky Fingers]

Using the rule of thumb formula as posted above, if your waterline length is say 40' then the max would be about 8.4kts. So 8, or less.

 

[johnalison]

I understand that the formula also breaks down when the length/beam ratio exceeds a certain figure, which is why multihulls can go faster. I have heard about non wave-making hulls being designed for cruisers with a narrow displacement hull widening into the accommodation hull that just sits on the water, but the idea didn't seem to get very far.

 

[westernman]

Just on this topic. I plan to do some long journeys at displacement on my Targa 48. At what point would I become uneconomical? Assuming 8 or 9 knots is ok for economic cruising. IPS 600, D6 435 engines

You would probably get about twice the miles per gallon at 6 knots vs at 9 knots.

 

[Sticky Fingers]

Yes it's a rule of thumb, not an absolute. Fine, long vessels do better. Short fat square ones do less well. A planing motorboat is much more short and fat than a rowing skiff for example.

 

[dunedin]

Just on this topic. I plan to do some long journeys at displacement on my Targa 48. At what point would I become uneconomical? Assuming 8 or 9 knots is ok for economic cruising. IPS 600, D6 435 engines

There is no real substitute for actual measurement of consumption at different speeds.
May be able to get close by noting speed at different engine revs (from 6-10 kts) and using engine data on L/h at each rpm ?

 

[MisterBaxter]

I think different hulls shift from displacement to planing with more or less effort, depending on load, hull shape etc. Boats that are long and slim with very shallow draft and fairly flat bottoms get up on the plane with little fuss or effort and the fuel consumption curve is a lot less dramatic.

 

[westernman]

From 0 to sqrt(lwl in feet) knots, there is relatively little fuel consumption/mile increase for going faster.
Above that speed, your fuel consumption/mile increases a fair rate.

Hence my suggestion of slowing down to 6 knots which is about sqrt(lwl) for this boat.

Above 1.3xsqrt(lwl in feet) knots for a full displacement hull, the fuel consumption/mile goes through the roof - and it will not ever go much faster.
As it will also for a semi-displacement boat until it is up on the plane. But the fuel consumption will always be much higher than at 1.3xsqrt(lwl).

 

[PaulRainbow]

Just on this topic. I plan to do some long journeys at displacement on my Targa 48. At what point would I become uneconomical? Assuming 8 or 9 knots is ok for economic cruising. IPS 600, D6 435 engines

Everyone seems to have forgot about tides. Working hull speed out is all well and good, but the actual SOG will depend on tidal effects. If hull speed is 6 knots and you're in an area where the tides run at up to 2 knots (just for instance), SOG will vary from 4 to 8 knots.

Real World trumps paper figures here, so increase speed until you start to create excess wash/dig a hole in the water behind the boat and then slow down slightly.

 

[Ian_Edwards]

My Southerly 46 has theoretical hull speed of just over 9knots. It's definitly a displaced hull. At 6.5 to 7knots I use about 2L per hour, flat out at 9knots plus, it's more like 10L per hour.
But I have had it surfing at 13knots, that's just short bursts going down the face of a wave, with gravity assistance. I guess it must be planing at that speed, but it's not a speed I can get close to in flat water.
I often see motor yachts, hurrying along generating massive bow waves and big wake, and wonder why they are pushing the boat that hard, when slowing down a couple of knots would make a significant difference to their fuel consumption.

 

[Sticky Fingers]

10l / hour at 9kts. Seems optimistic but you may be right.

 

[PaulRainbow]

My Southerly 46 has theoretical hull speed of just over 9knots. It's definitly a displaced hull. At 6.5 to 7knots I use about 2L per hour, flat out at 9knots plus, it's more like 10L per hour.
But I have had it surfing at 13knots, that's just short bursts going down the face of a wave, with gravity assistance. I guess it must be planing at that speed, but it's not a speed I can get close to in flat water.
I often see motor yachts, hurrying along generating massive bow waves and big wake, and wonder why they are pushing the boat that hard, when slowing down a couple of knots would make a significant difference to their fuel consumption.

Exactly what i meant in post #17 Ian